JPS6210447B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an isolated digital signal input circuit, in which an input signal is converted into a short-circuit open circuit in the primary winding of an isolation transformer by switching transistors, and a short-circuit open circuit in the primary winding of an insulating transformer is generated by an input pulse in the secondary winding. It converts the state of the line into an internal signal.

In process control equipment, etc., when reading signals generated in the form of contacts on the process side, the presence or absence of on/off voltage of transistors, etc. into the control equipment, there is It is common to provide insulation.

Insulation means include relays, photocouplers, transformers, etc., and various circuit systems can be considered, but it is sometimes desirable to isolate multi-point signals using simple circuits that are inexpensive, fast, and space efficient. A purpose is set.

First of all, as a general concept, this type of device is
AD isolated from control device 1 as shown in the figure.
A signal from a device 2 such as a converter is DC-isolated from the control device 1 via an isolation circuit 3, captured into a register 4 in the control device 1, and converted into a synchronized internal signal. The configuration is such that the data is sent to the arithmetic control unit 5 for processing and process control.

Conventionally used insulation methods include the photocoupler insulation method shown in FIG. 2a, the relay insulation method shown in FIG. 2b, and the insulation transformer method shown in FIG. 3.

Among these insulation methods, photocoupler insulation is the one that connects the light emitting diode 6a and the phototransistor 6b.
This device insulates the primary side A and the secondary side B using a photocoupler 6 typified by , and when the light-emitting diode 6a is lit by the current flowing from the power source 8 through the resistor 9 by opening and closing the contact 7, the phototransistor 6b is turned on. When it is turned on, its output is taken in through the inverter 10 and into the flip-flop 11 in synchronization with the pulse RD, thereby producing an isolated output UT.

Although this method is relatively easy to use, since the photocoupler itself is a new element, there are concerns about reliability regarding changes in transmission efficiency over time.
Also, the operating speed is slower than that of the internal circuit.

Next, the relay method is a case where a relay 12 is used as shown in FIG. 2b in place of the photocoupler 6 in FIG. 2a, and the current flowing through the coil 12a causes the contact to
2b is closed, the signal is transmitted, and the isolated output UT
We take it out. In this case, the response speed is slow and the reliability is low because it is a mechanical contact.

Finally, the transformer method transmits the opening and closing of the contact 7 via a transformer 13, as shown in FIG. Since direct current changes such as opening and closing of the contact 7 cannot be transmitted by a transformer, the primary winding of the transformer is short-circuited or opened according to the input signal, and a pulse is applied to the secondary winding. An output pulse corresponding to the short-circuit or open state of the primary winding is output to the secondary winding. This is a specific circuit that was previously proposed by the applicant in Japanese Patent Application Laid-Open No. 110398/1983 and has been put into practical use.

In the figure, a transistor is installed on the primary side of the transformer 13.
A Tr, a Zener diode Z, and resistors R ₁ and R ₂ are additionally used. Also, on the secondary side, the intake pulse
RD is applied to one end a of the secondary winding 13b of the transformer 13 through the NAND gate 16, and the same winding 13b
The other end b passes through a flip-flop 11 and the UT output is taken out. Also, flip-flop 11
A clock can be connected to the CK terminal. Other power supplies
Vcc passes through impedances 14 and 15 to point b, a
added to the point.

When the contact 7 is closed, the current supplied from the power source 8 passes through the circuit of resistor R ₁ →Zener diode Z →transistor Tr, becomes the base current of transistor Tr, and turns on transistor Tr. In this state, if the rectangular take-in pulse RD shown in FIG. A pulse signal shown in FIG. 4b appears at point b at one end of the next side 13b. Then, the clock signal shown in FIG. 4c is applied to the clock terminal of the flip-flop 11.
When CK is added, the signal at point b mentioned above is latched as an "L" level signal.

Next, if contact 7 is open, the transistor
Tr is in a cut-off state. When the take-in pulse RD at point a is turned off, a voltage with the _polarity shown in FIG. A current iBP flows through the collector, and a current iTP, which is amplified from the current iBP, flows from the emitter to the collector, and the transistor Tr becomes conductive. In other words, transformer 1
Since the primary side 13a of the transformer 13 is short-circuited, it takes time to reset the excitation current of the transformer 13, and it does not respond to the intake pulse within a short time interval.

Also, when the take-in pulse is turned on in this state, a signal that turns on instantaneously is output as shown in Figure 4d.
The signal acquisition timing is difficult. Also the fifth
If a diode VD is added to the collector circuit of the transistor Tr as shown in Fig. 5b to prevent the circuit shown in Fig. The signal rises above the "L" level by VBE, and the margin for the input signal level of register 4 in Figure 1 decreases, requiring an amplifier with a sufficiently stable threshold level in front of register 4. .

As mentioned above, the transformer method is compact and high-speed, but it has a problem with noise margin and requires some ingenuity in the circuit.

In consideration of the above points, the present invention provides a transformer type digital signal isolation circuit which has a simple circuit configuration, is high speed, and has no timing problems. That is, in this invention, as an improvement of FIG. 3, the threshold voltage of the input signal is determined by inserting a Zener diode between the emitter and ground of the emitter-grounded transistor switch circuit, and the threshold voltage of the secondary winding is This is designed to prevent the influence of the capture pulse.

Therefore, except for the insertion of the Zener diode, the circuit shown in FIG. 6, which is an embodiment of the present invention, is the same as that of FIG. 3, and there is no problem.

In Fig. 6, when the contact 7 is closed, a current flows from the power supply 8 through the contact 7 and the resistors R ₁ and R ₂ .
When the voltage across the resistor R ₂ becomes the sum of the Zener voltage (V _Z ) and the base-emitter voltage V _BE of the transistor Tr, the transistor Tr is turned on. Further, when the contact 7 is opened, the transistor Tr is turned off.
The threshold voltage of this circuit is (V _BE +V _Z ) (1+R ₁ /R ₂ ), and the input threshold level can be determined within a sufficiently stable range for a digital input circuit. When the transistor Tr is on, the input pulse RD in FIG.
When the voltage is applied to point a at one end of the secondary side of the transformer 13, the seventh voltage is applied to the other point on the secondary side of the transformer 13.
The signal shown in FIG. 7B is generated and taken in as an "L" level signal by the clock signal shown in FIG. 7D to the clock terminal of the flip-flop 11.

Further, when the transistor Tr is off, if the take-in pulse RD shown in FIG. 7a is applied to the point a mentioned above through the gate 16, the signal shown in FIG. The clock signal shown in FIG. 7d is taken in as an "H" level signal.

In this case, in this invention, the Zener diode Z is inserted between the emitter of the transistor Tr and the ground, so the equivalent circuit shown in FIG. 5a is not formed.
In addition, there is no voltage drop due to the diode V _D shown in Figure 5b, and the input signal is sufficiently stable at both the "L" and "H" levels, making it suitable for general TTL (transistor) input signals.
(transitor logic) signal level.

As described above, according to the present invention, since the Zener diode Z is inserted between the emitter of the input side transistor Tr and the ground, the setting of the threshold voltage of the input signal level from the external circuit is stable. It is possible to prevent the timing of the next-side intake pulse OFF from being delayed, and effects such as stable signal level transmitted to the secondary side and easy processing can be obtained.

In short, this invention includes a primary winding, a secondary winding,
By installing an isolation transformer whose primary winding is short-circuited or opened according to the input signal, and by applying a capture pulse to the secondary winding, the secondary winding can be short-circuited or opened according to the short-circuit or open state of the primary winding. A digital signal isolation circuit for extracting output pulses, which uses a transistor switch circuit between the emitter and ground to short-circuit or open the primary winding, and inserts a Zener diode between the emitter and ground.
An isolated digital signal input circuit is obtained which determines the threshold voltage of an input signal and prevents the influence of the input pulse of the secondary winding.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a general process control device, Figures 2a and b are wiring diagrams of different conventional insulation circuits,
Fig. 3 is a wiring diagram of the previously proposed transformer type insulation circuit, Figs. 4 a to e are waveform diagrams for explaining the operation at predetermined positions in Fig. 3, and Figs. 5 a and b are the circuits in Fig. 3. An explanatory wiring diagram, FIG. 6, is a transformer type insulation circuit wiring diagram according to the present invention, and FIGS. 7 a to d.
is a voltage waveform diagram for explaining the operation at a predetermined position in FIG. 6; In the figure, 7: contact, 8: power supply, Tr: transistor switch, R ₁ , R ₂ : resistor, Z: Zener diode, 11: flip-flop, 16: NAND gate, 13: transformer.

Claims (1)

[Claims] 1 Equipped with an isolation transformer having primary and secondary windings,
Digital signal isolation in which the primary winding is short-circuited or opened according to the input signal, and by applying a take-in pulse to the secondary winding, an output pulse corresponding to the short-circuit or open state of the primary winding is taken out to the secondary winding. In the circuit, in order to short-circuit or open the primary winding, a transistor switch circuit with a grounded emitter is connected in parallel to the primary winding, and a Zener diode is connected between the connection point of the emitter and the primary winding and the ground. An isolated digital signal input circuit characterized in that a digital signal input circuit is inserted into the circuit.